is characterized by mesangial IgA1 immune deposits that originate from circulating immune complexes containing aberrantly-glycosylated IgA1, i.e., IgA1 with galactose (Gal)-deficient O-glycans. Several lines of evidence suggest a direct causal relationship between aberrant glycosylation, the formation of immune complexes containing aberrantly glycosylated IgA1, their deposition in the mesangium, and renal injury in IgAN. We determined that IgA1-producing cells of IgAN patients secrete IgA1 with Gal-deficient O- glycans and that this aberrancy is due to dysregulation in expression/activity of specific glycosyltransferases. These findings indicate that Gal-deficient IgA1 plays a pivotal role in the pathogenesis of IgAN. An understanding of the molecular basis for the variations in the carbohydrate content of IgA1 in IgAN is essential for the definition of the fundamental defects that result in the synthesis of the aberrant glycans in IgAN. Such studies have been frustrated, however, by the fact that it is not feasible to obtain sufficient cells of interest from human mucosal tissues, lymph nodes, or bone marrow and by the lack of animal models, as IgA1 is present exclusively in humans and hominoid primates. As a first step toward developing a murine model, we have now shown that immune complexes, prepared in vitro between human Gal-deficient IgA1 and anti-glycan IgG and i.v.-injected to nude mice, deposit in the mesangium and induce hematuria and proteinuria. Because mice do not have IgA with hinge-region O-glycans, we hypothesized that generating transgenic mice with IgA containing the human hinge region would result in murine IgA containing O-linked glycans that can be manipulated to resemble the aberrant human IgA1. Such a transgenic mouse strain would represent a new tool for studies of the role of the O-glycans in IgAN and in mucosal immunity. We propose to generate a knock-in transgenic mouse strain producing IgA with hinge region from human IgA1, assess its O-glycosylation, and, by inhibiting the key enzyme - 21,3-galactosyltransferase, to generate Gal-deficient O-glycans on the murine transgenic IgA. Furthermore, we will determine whether the immune complexes composed of murine transgenic Gal-deficient IgA and glycan-specific IgG deposit in renal mesangium of mice causing nephropathy. We will further develop and validate this first animal model truly reflecting the human disease by performing histological analysis of renal tissue and laboratory analysis of urine and serum. This transgenic mouse will open new possibilities for testing genetic and biochemical mechanisms involved in production of aberrantly O- glycosylated IgA and its mesangial deposition and clearance. Relevance: IgAN is the most common primary glomerulonephritis and leads to end-stage renal failure in 20% to 40% of patients. The current gaps in the understanding of the pathogenesis of IgAN represent a major barrier to the development of IgAN-specific treatments. The proposed studies will provide the foundation for the future development of a relevant physiological animal model of IgAN that will advance the understanding of the pathogenesis of human IgAN. PUBLIC HEALTH RELEVANCE: We propose to develop a murine model of IgAN. We will generate transgenic mice in which IgA will include the human hinge region containing O-linked glycans. We will further generate Gal-deficient O-glycans on the murine transgenic IgA and use anti-glycan IgG to form pathogenic complexes that will deposit in the kidneys.